This invention relates to a chemically bonded refractory shape, having particular utility in any steel-making plant (except an open hearth furnace) which has an external metal jacket which prevents air from being absorbed into the cold face of the refractory, such as a basic oxygen furnace (BOF), electric furnace, argon-oxygen degassing unit (AOD), and vacuum induction melting furnace (VIM). The refractory shape of the present invention exhibits a synergistic improvement in resistance against slag erosion, together with increased modulus of rupture, crushing strength and oxidation resistance, by reason of additions of both particulate metallic magnesium and particulate elemental carbon to the refractory mix.
Pitch-bonded, hot formed refractory shapes (e.g. in the form of bricks) wherein the refractory material is magnesia, doloma, or alumina, and the pitch is derived from coal tar or petroleum, have found increasing use in lining vessels used for refining steel. Various proposals and additives have been suggested by the prior art for improving resistance against erosion by slag, molten metal, and hot gases.
The present invention constitutes a further improvement in the desired properties, particularly erosion resistance, of refractory shapes of the above type for use in steel refining vessels.
U.S. Pat. No. 2,013,625 discloses a refractory article having a vitreous protective surface in which the refractory aggregates are bonded by residual carbon (which may be derived from tar, pitch, resin, molasses, or dextrin), and in which a metallic substance (such as iron, manganese, magnesium, copper, lead, zinc, or alloys) is dispersed throughout the article.
U.S. Pat. No. 2,741,822 discloses a shaped refractory article comprising a fused mass of a single refractory oxide (alumina, magnesia, or silica) and granulated metal (aluminum, magnesium, or silicon) in a ratio of about 10 parts oxide to 1 part metal, the shaped article then being oxidized at about 1000.degree.-1200.degree. C. to oxidize and fuse the metal particles substantially completely. A volatile organic bonding agent is also mixed initially with the metallic oxide.
U.S. Pat. No. 3,226,456 discloses the production of increased density ceramic articles by mixing with a metal oxide about 0.02% to 0.1% by weight of the same metal in particulate form ranging in size between about 1 and 40 microns, followed by shaping at a temperature up to about 1825.degree. C. and molding under pressure. Metals which may be used are aluminum, beryllium, magnesium, thorium and zirconium. The metal is at least partially oxidized in situ during the shaping and heating step.
U.S. Pat. No. 3,322,551 discloses a pitch-bonded, basic refractory to which is added from 0.5% to 1.5% by weight of the refractory batch of finely divided aluminum or magnesium. When pitch having a softening point above 200.degree. F. was used (in an amount of 1% to 10% of the total batch weight) as the bonding agent, an improvement in erosion resistance was obtained under conditions simulating an oxygen steel converter. The metal particle size was less than 0.210 mm (all passing a 65 mesh screen), and preferably less than 0.149 mm (all passing a 100 mesh screen).
German OLS No. 3004712, published Aug. 21, 1980, in the names of Kyusyu Refractories, discloses the production of unfired refractory carbon bricks by adding from 1% to 10% aluminum and/or magnesium powder to a refractory material containing greater than 1%, and especially from 5% to 75% carbon. The particle size of the metal powder is preferably less than 0.125 mm. From 0.5% to 6% silicon powder may also be added to prevent the hydration of carbides. Increased resistance to oxidation and reduced porosity are alleged to result from the aluminum and/or magnesium addition.
The formation of a dense zone of magnesium oxide just behind the hot face of pitch bonded magnesia or doloma brick during service in BOF or AOD vessels is disclosed in articles co-authored by the present applicant, viz. "Microstructural and Chemical Changes of Pitch Impregnated Magnesite Brick under Reducing Conditions", Trans. British Ceramic Society, Vol. 71, No. 6, pp. 163-170, Sept. 1972, B. Brezny and R. A. Landy; and "Role of Carbon in Steel Plant Refractories.", The American Ceramic Society Bulletin, Vol. 55, No. 7, pp. 649-654, July 1976, B. H. Baker, B. Brezny and R. L. Shultz. A discussion of this phenomenon also occurs in U.S. Pat. No. 4,196,894 issued Apr. 8, 1980 to the present applicant. Briefly summarized, it appears that in the carbon zone of the brick magnesium oxide is reduced to metallic magnesium by the carbon in the pitch bond, during service. The metallic magnesium is vaporized and migrates toward the hot face of the brick. In the region just behind the hot face the magnesium is oxidized back to magnesium oxide and is precipitated there to form a dense zone. Any voids originally present are filled by the magnesium oxide precipitate, preventing penetration by slag, hot metal and/or gases, and therefore decreasing erosion.
Although not intending to be bound by theory, it is believed that the same phenomenon as that described above occurs in the refractory shape of the present invention during service. However, this invention constitutes the further discovery that a marked improvement in erosion resistance occurs when both particulate elemental carbon and particulate elemental magnesium are added to a refractory mixture having an organic binder, this improvement being of far greater magnitude than that obtained by adding particulate elemental magnesium alone to a pitch bonded refractory, and that obtained by adding particulate elemental carbon alone to a pitch bonded refractory. The addition of carbon and magnesium in combination thus produces a true synergistic effect, which is substantiated by test data hereinafter set forth.
It has been found that the markedly improved erosion resistance, which is the principal object of the present invention, is achieved by use of an anhydrous refractory material, an anhydrous organic binder, at least 0.3% finely divided elemental carbon having a particle size between 0.01 and about 1 mm and at least 0.3% particulate elemental magnesium, based on the total weight of the refractory mix. Residual carbon derived from the binder as a result of coking (which occurs during service) has been found to be ineffective in producing marked improvement in erosion resistance regardless of the amount of magnesium added.
Where a brick of conventional type having about 5% anhydrous organic binder (such as pitch) is being produced in accordance with the invention, the elemental carbon addition ranges from 0.3% to 3% by weight and the elemental magnesium addition from 0.3% to 3.5% by weight. There is also presently available brick having carbon additions up to 25% to even 30% for high thermal conductivity. The brick of the present invention can be produced at carbon levels up to 30% by weight if flake graphite having a particle size of about 1 mm is added, and in this embodiment up to about 10% elemental magnesium is added.
According to the invention there is thus provided a refractory shape having improved resistance to erosion, comprising an anhydrous refractory material, an anhydrous organic binder in an amount sufficient to bond said refractory material, from 0.3% to about 30% finely divided elemental carbon having a particle size ranging from 0.01 to about 1 mm, and from 0.3% to about 10% particulate elemental magnesium, based on the total weight of the refractory shape.